Substrate with terminal pads having respective single solder bumps formed thereon

ABSTRACT

Methods and apparatus for forming solder bumps on terminal pads of a semiconductor substrate for an integrated circuit device employ a solder bump transfer plate and a mask to form solder deposits on the plate. One embodiment of the invention employs a metal mask having a plurality of through holes for forming solder deposits on the solder bump transfer plate by vapor phase deposition through the through holes each area of which increases in step wise from the first surface of the mask to the second surface opposite to the first surface, thereby preventing solder deposits in the through holes from being removed when the mask is separated from the plate. Another embodiment of the invention is a solder bump transfer plate having a plurality of solder deposits on the surface non-wettable to molten solder both diameter and spacing of which are both smaller than diameter and spacing of the terminal pads on the semiconductor substrate, whereby a single solder bump is accurately formed on each of the terminal pads without a fine alignment technique.

FIELD OF INVENTION

The present invention relates to an integrated circuit device, moreparticularly to methods of transferring solder bumps onto an integratedcircuit device, such as a flip chip semiconductor device, and to anapparatus for transferring the solder bumps, such as a solder bumptransfer plate or a metal mask for forming solder deposits on the plate.

BACKGROUND OF THE INVENTION

It is known that a semiconductor chip having an array of terminal padson a chip surface is mounted on a printed circuit board or anothersemiconductor chip also having an array of terminal pads by flip-chipmethod, wherein the arrays of terminal pads on a chip are connected witheach other by vertical solder bump interconnections between a chip and aprinted circuit board or another semiconductor chip. For typicalprocessing, solder bumps are transferred from a solder bump transferplate to each of terminal pads on a chip surface. Solder deposits on asolder bump transfer plate are usually formed on a glass substrate byvapor phase deposition with a metal mask or by selective electroplatingmethod. Generally, as packing-density of integrated circuits increases,both size and space of terminal pads are needed to decrease, from whichvarious technical problems arise, such as solder bridges connectingbetween adjacent terminal pads causing short circuit between theterminal pads, or non-uniformity of solder amount application per padscausing electrical disconnection between a vertical solder bumpinterconnection. Solder deposits, which are predecessors of solderbumps, on a solder bump transfer plate formed by vapor phase depositionthrough through-holes of a metal mask are often detached from depositedsites when the metal mask is separated from the solder-bump transferplate, because the solder deposits are often adhering to inside walls ofthe through-holes. In Japanese Laid-open Patent Application No.5-235003, a method is described that an inside wall of a through-hole ofa metal mask is lined with material having non-wettable tendency tomolten solder. In this method, however, repelled solder is solidifiedaround a solder bump in cooling as solder bridges or solder balls whichoften cause short circuit between terminal pads adjacent to each other.A solder ball is usually produced on a surface of a semiconductor chipbetween terminal pads from an excessive solder extending to theoutskirts of a solder deposit deposited on a solder bump transfer plateusing a metal mask when the solder deposit is melt to transfer onto theterminal pad. As an attempt to remove the solder ball described inJapanese Laid-open Patent Application No. 63-261857, photosensitivepolyimide film is formed on the whole surface of a semiconductor chipexcept terminal regions and solved by organic solvent later. However,this method is incompatible to semiconductor chips having polyimide asan insulating film.

SUMMARY OF INVENTION

It is an object of the present invention to provide a solder bumptransfer device for transferring solder bumps onto terminal pads of asemiconductor device without a severe aligning requirement.

It is another object of the present invention to provide a solder bumptransfer device for transferring solder bumps onto terminal pads havinga fine size and a narrow spacing on a semiconductor device withoutleaving solder bridges or solder balls between the terminal pads.

It is a further object of the present invention to provide a method fortransferring solder bumps having a uniformity in height and strengthonto terminal pads of a semiconductor device.

It is a still further object of the present invention to provide a maskfor forming solder deposits on a surface of a substrate or a plate byvapor phase deposition through through-holes of the mask and for beingremoved easily without detaching the solder deposits in thethrough-holes.

In one aspect of the present invention, both diameter and spacing ofsolder deposits on a solder bump transfer plate are smaller thandiameter and spacing as well of terminal pads on a semiconductor device,whereby a single solder bump is formed on each of the terminal padswithout a severe aligning requirement. Needless to say, ancross-sectional area of the solder deposits and an area of the terminalpads are not necessarily limited to a circle. In another aspect of thepresent invention, the whole surface except terminal pads of asemiconductor device is coated with material non-wettable to moltensolder which is removed later together with solder balls remainingthereon. In further aspect of the present invention, a mask hasthrough-holes each diameter of which increases in step wise from thefirst surface of the mask to the second surface opposite to the firstsurface, thereby, after solder deposits are deposited through thethrough-holes on a surface of a solder bump transfer plate against whichthe second surface of the mask is pressed, the mask is easily removedwithout detaching a solder deposit in a through-hole.

The techniques according to the present invention may be applicable toany planar surface of a substrate to form a plurality of solder bumpsthereon, and to stacked flat plates interconnected by solder bumpstherebetween.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the invention are described with reference tothe accompanying drawings, in which:

FIGS. 1A through 1D are diagrammatic section views of a pair of a solderbump transfer plate and a semiconductor substrate in various processingsteps related to the first embodiment in accordance with the presentinvention.

FIGS. 2A through 2F are diagrammatic section views of a pair of a solderbump transfer plate and a semiconductor substrate in various processingsteps related to the second embodiment in accordance with the presentinvention.

FIGS. 3A through 3C are diagrammatic section views of a solder bump on asemiconductor substrate in various processing steps related to the thirdembodiment in accordance with the present invention.

FIGS. 4A through 4D are diagrammatic section views of a pair of a solderbump transfer plate and a semiconductor substrate in various processingsteps related to the fourth embodiment in accordance with the presentinvention.

FIGS. 5A through 5D are diagrammatic section views of a pair of a metalmask and a solder bump transfer plate in various processing stepsrelated to the fifth embodiment in accordance with the presentinvention.

FIG. 6 is a diagrammatic section view of a pair of a metal mask and asolder bump transfer plate related to the sixth embodiment in accordancewith the present invention.

FIGS. 7A through 7D are diagrammatic section views of a solder bumptransfer plate, a metal mask, and a semiconductor substrate in variousprocessing steps related to the seventh embodiment in accordance withthe present invention.

FIG. 8 is a graph showing the height distribution of solder bumps acrossa semiconductor chip fabricated by a single transferring operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1A, a solder bump transfer plate 1 is a glass plate asurface on which a plurality of solder deposits 11 made of Indium (In)alloy with 48 wt % Tin (Sn) are formed in a matrix having a spacing of200 μm and a diameter of 100 μm by screen printing method. A substrate 2is an alumina ceramic circuit board for a hybrid IC a surface on which aplurality of terminal pads 21 made of 0.1 μm thick electro-plated goldon 0.5 μm thick nichrome (hereafter noted by 0.1 μm thick Au/0.5 μmthick Ni) are arranged in a matrix having a spacing of 800 μm and adiameter of 400 μm.

Referring to FIG. 1B, after solder flux 4 is applied, the solder bumptransfer plate 1 is positioned on the substrate 2 without a finealignment such that the same number of the solder deposits 11 rest oneach of the terminal pads 21.

Referring to FIG. 1C, an assembly of the plate 1 and the substrate 2 isheated to 150° C. for 2 minutes in a furnace such that the solderdeposits resting on a terminal pad melt to be transferred onto theterminal pad to form a single solder bump 3, and solder deposits notresting on a terminal pad melt to form solder balls 31 in solder flax 4.

Referring to FIG. 1D, after the assembly is cooled, the substrate 2 isseparated from the plate 1 and washed to removed the solder fluxtogether with the solder balls therein, then an alumina ceramic circuitboard having a single solder bump on each of the terminal pads isobtained.

The above example can be modified as follows:

A solder bump transfer plate 1 is a polyimide film, solder deposits 11made of Indium alloy are formed in a matrix having a spacing of 100 μm,a diameter of 50 μm, and height of 50 μm by employing a metal mask (notshown). Terminal pads 21 made of 0.1 μm thick Au/0.5 μm thick Ni arearranged in a matrix having a spacing of 500 μm and a diameter of 200μm. After solder flux 4 is applied, the solder bump transfer plate 1 ispositioned on the substrate 2 without a fine alignment, and pressed at150° C. with 5 kgf such that the solder deposits 11 resting on theterminal pads 21 are thermally bonded to the terminal pads. The assemblyis heated to 220° C. such that the solder deposits resting on a terminalpads are transferred onto the terminal pad to form a single solder bump3, the rest of process is substantially the same as that of the firstexample, wherein a substrate may be Si chip, solder alloy may be Bi, Ga,Ge, Sb, or Pb-63% Sn other than or In-48% Sn.

It should be noticed that since spacing and diameter of the solderdeposits 11 are both smaller than those of the terminal pads 21 in theseexamples, no fine alignment of solder deposits to terminal pads isrequired.

Referring to FIG. 2A, a solder bump transfer plate consists of a Sisubstrate 1 and Pb-5% Sn solder deposits thereon. The solder depositsare deposited through a metal mask (not shown) having through-holes of130 μm in diameter at the first surface and 170 μm in diameter at thesecond surface opposite to the first surface by pressing the secondsurface against the Si substrate 1, where an inside-wall of thethrough-holes is sloped by an angle of 100° from the first surface. Adeposit 111 on the most right site is intentionally drawn smaller thanthe others 11 having a height of 30 μm. A Si substrate 2 for integratedcircuits has a plurality of terminal pads 21 which has the same diameterand spacing as those of the solder deposits, respectively in thisembodiment.

Referring to FIG. 2B, after solder flux is applied, the solder bumptransfer plate 1 is positioned on the Si substrate 2 such that each ofthe terminal pads is aligned to a corresponding one of the solderdeposits and that all of the aligned solder deposits including thesmaller solder deposit 111 are in contact with the correspondingterminal pads. The solder flux is applied to the surface of the solderbump transfer plate 1 as same as before.

Referring to FIG. 2C, an assembly of the plate and the Si substrate isheated at 360° C. such that each of the solder deposits is transferredonto the corresponding terminal pad to form a single solder bump on eachof the terminal pads after separating the plate from the Si substrateand washing away the solder flux.

When the assembly is heated, the solder deposit is melt to transformitself into a droplet of molten solder. Transferring the droplet ofmolten solder onto the corresponding terminal pad probably arises fromcollective effects of a gravity, a wettable tendency of the terminal padto molten solder, a non-wettable tendency of the glass plate to moltensolder, and a surface tension of the droplet. Therefore, the solidsingle solder bump maintains a spherical shape covering the entirewettable surface of the metalized terminal pad. A specific single solderbump transferred from the shorter deposit 111 inevitably has a heightlower than those the others have. This would cause disconnection of avertical interconnection if the semiconductor substrate would be mountedon a printed circuit board by flip-chip method as it is. The main reasonfor non-uniformity of a bump height is result from non-uniformity of adiameter of through-holes in a metal mask which is originated from.

FIG. 8 is a graph of height distribution of solder bumps across a chipmade by a single transferring operation, which indicates that solderbumps having heights deviated largely denoted by a solid arrow for ataller bump and an empty arrow for a shorter bump, for example, by morethan 10% of the average value, are quite few. Since a taller bump can bedeformed at flip-chip connection, it is not needed to be considered as adetective bump. For a practical use, no disconnection is found out forsolder deposits having height not less than 90% of the average value.Thus, the disconnection problem caused by a shorter bump can be avoidedby repeating the entire processing steps from forming solder deposits ona glass plate to transferring solder bumps onto metalized terminal padsof a semiconductor chip. For example, if it is repeated twice, thesolder deposits for a single deposition may have half a volume of thefinally required volume.

Although a certain precaution is needed to prevent a shorter solderdeposit on a bump transfer plate from being systematically aligned toanother shorter solder bump at an identical specific site on asemiconductor substrate, if a through-hole of a metal mask has adiameter smaller by 20% than the average value at the rate of {fraction(1/10,000)}, the probability that two smaller bumps will meet with eachother is less than ({fraction (1/10,000)})² which is practically anegligible small value. Thus, according to the twice repetition method,for instance, if a semiconductor device has 3,000 terminals on a chip,the disconnection will occur at the rate of less than one out of 30,000units, while it will occur at the rate of one out of three units by theconventional method. The repetition numbers increase, the defective ratedecreases sharply.

The finally required bump height is obtained by twice-repetition ofsolder deposition, wherein height of a solder deposit is one-half of thesolder deposit by a single deposition which will give the finallyrequired bump height. Since relative volumes of two cylindrical solderdeposits deposited through a circular through-hole having a normaldiameter and another circular through-hole having a diameter smaller by20% than the normal one are 0.5 and 0.5×(1−0.2)², namely 0.32,respectively, a spherical solder bump made by the above two solderdeposits will have a bump height of (0.5+0.32)^(1/3), namely 0.94 whichis within ±10% tolerance.

Referring to FIG. 2D, for the reason discussed above, a metal maskhaving a through-hole whose diameter is less than 80% of the averagevalue is removed as a defective unit in mask inspection.

Referring to FIG. 2E, subsequently, the steps are repeated so as totransfer another solder bump onto each of the single solder bumpsalready made on the terminal pads by the previous steps.

Referring to FIG. 2F, after separating the plate, and washing away thesolder flux, the Si substrate 2 is obtained which has a single solderbump on each of the terminal pads whose height is higher than that ofthe first single solder bump and that an error of the height will beless than 10% of the average value.

An application of the above repetition method to a Si substrate showedan average height of the solder bumps 84.3 μm high, the minimum height87.9 μm high, and the maximum height 87.9 μm high. By employing this Sisubstrate, a CPU module is assembled with a nitric aluminium circuitboard by flip-chip bonding method without flux wherein no defective unitis found out at electric testing in vertical interconnections. Similarexperimental data are summarized in Table 1 and 2, where Table 1 showsheating temperatures for various bump solders and Table 2 shows bumpheights for the various bump solders. TABLE 1 Transfer DeoxidizedBonding Bump solders temperature temperature temperature Pb-63 wt % Sn220° C. 210° C. 260° C. In 200° C. 180° C. 260° C. In-34 wt % Bi 200° C.150° C. 260° C.

TABLE 2 Average Minimum Maximum Bump solders height height height Pb-63wt % Sn 83.8 μm 79.1 μm 87.5 μm In 84.1 μm 79.6 μm 88.5 μm In-34 wt % Bi84.0 μm 78.0 μm 87.4 μm

As an example, by employing a metal mask having through-holes of anominal diameter 150 μm on the first surface and a nominal diameter 180μm on the second surface with an inside wall of a tapering angle 100°from the first surface which actually has an average diameter 150 μm,and the minimum diameter larger than 125 μm on the first surface,cylindrical solder deposits of Pb-5 wt % Sn were formed on a solder bumptransfer glass plate to get spherical solder bumps of an average height85 μm and the minimum height 75 μm, and then transferred ontoNi-metalized terminal pads on a Si substrate at 360° C. This transferprocessing step was repeated twice. The following measurement of bumpheights revealed that an average height of 84.9 μm, the minimum heightof 78.3 μm, and the maximum height of 90.1 μm. The Si substrate withthese bumps was mounted on an AlN substrate by flip-chip method tocomplete a CPU module. Electrical reliability tests on these devicesgave a result that no defective units were found out. Particularly, itwas found out from various reliability tests that a hourglass shapedsolder bump connecting both substrates at terminal pads is moredesirable than a barrel shaped solder bump, because a thermal stress mayeasily concentrate on an interface between a solder bump and theconnecting terminal pad, while easy inelastic deformation of a solderbump near the middle point would absorb the thermal stress.

From these experiments, it has been assured that a combination ofselecting a metal mask by inspecting through-holes such that the minimumdiameter of the through-hole is determined in advance and of repeatingtransfer of solder bumps is effective to equalizing the final height ofthe solder bumps.

A method for fabricating a flip-chip device comprising two majorprocessing steps is effective to achieve a remarkable result in higherreliability in electric interconnections, wherein the two majorprocessing steps are firstly to select a metal mask by inspectingthrough-holes such that an acceptable mask has through-holes whoseopening area has a predetermined minimum limit in unavoidable deviationfrom the average value, and secondly to repeat the solder bump transferprocess until every bumps reach a finally required height on metalizedterminal pads of a substrate. The method effectively eliminatesdefective units having disconnection failure.

Referring to FIG. 3A, a basic part of the method for forming solderbumps on terminal pads on a semiconductor substrate for this embodimentwas the same as that as shown in FIGS. 2A through 2C. However, thesemiconductor substrate 2 had an insulating layer 22 on an entiresurface except the terminal pads 21, and also had a metal pattern 6partly on the terminal pads 21 and partly on the insulating layer 22such that a surface of the corresponding terminal pad was partlyexposed. The terminal pads 21 was metalized by nickel which may bereplaced by other metals wettable to molten solder such as Au, Ti, Cu,Cr or any combination of these. The insulating layer 22, which waspolyimide, was non-wettable to molten solder. The metal pattern 6 was0.5 μm thick gold layer and had essentially the same diameter as that ofthe solder deposit. The metal pattern 6 was wettable to molten solderand easily melts into the molten solder. In the step of positioning thesolder bump transfer plate on the semiconductor substrate 2, each of thesolder deposits 11 was aligned to, and in contact with the metal pattern6 which was already aligned eccentrically from the correspondingterminal pad 21.

Referring to FIG. 3B, by heating, the solder deposit was melt into asolder droplet 11 resting on the metal pattern 6 in an early stage, andsubsequently the metal pattern 6 was also melt into the solder droplet.Melt-down of the metal pattern 6 made the solder droplet contact withboth the insulating layer 22 and the terminal pad 21 which resulted in arepulsive force to push the solder droplet to the terminal pad 21, andsimultaneously an attractive force to pull the solder droplet into theexposed surface of the terminal pad 21 as indicated by an arrow. A factthat a surface of the insulating layer was higher than that of theterminal pad also assisted the solder droplet to move to a center of theterminal pad by gravitational force.

Referring to FIG. 3C, at the final stage, a surface tension made thesolder droplet spherical resting on the entire surface of the terminalpad 21 within an opening of the insulating layer, and by cooling, thesolder droplet was solidified into a solder bump 3 which maintained aspherical shape concentric with the terminal pad 21.

Referring to FIG. 4A, a solder bump transfer plate 1 had solder deposits11 each of which usually had a tapering part 32 around the solderdeposit. The tapering part 32 was, more or less, concomitantly formed bydepositing Pb-5 w % Sn solder deposits of 30 μm high through a metalmask (not shown) by vapor phase deposition. The metal mask hadthrough-holes of 130 μm in diameter at the first surface and 170 μm indiameter at the second surface opposite to the first surface. Thedeposition was carried out by pressing the second surface against the Sisubstrate 1, where an inside wall of the through-holes was tapered by anangle of 100° from the first surface. A semiconductor substrate 2 hadterminal pads 7 on the surface and a 0.1 μm thick aluminium layer 21therebetween. The terminal pads 7 was metalized by gold. The aluminiumlayer 21 was non-wettable to molten solder.

Referring to FIG. 4B, after applying solder flux, the solder bumptransfer plate was positioned on the substrate 2 by aligning the solderdeposits to the gold metalized terminal pads, and then the assembledsubstrate was heated at 360° C. until each of the solder deposits weremelt into a single solder droplet on the corresponding terminal pad.

Referring to FIG. 4C, after cooling the assembled substrate, and thenseparating the glass plate 1 from the substrate 2, a solidifiedspherical single solder bump 3 was formed on each of the terminal padsand solder balls 31 were left on the aluminium layer 21 with remainderof the solder flux 11.

Referring to FIG. 4D, by washing away the solder flux and then immersingthe substrate into an etchant consisting of 90 ml water, 15 ml HCL, and10 ml HF to remove the aluminium layer 21 and solder balls 31 together,the substrate 2 having the single solder bumps on the terminal pads 7was finally obtained.

As a non-wettable layer to molten solder formed on an entire surfaceexcept terminal pads, heat resistant polymer like polyimide is easilyremoved by basic solution, but for a substrate already employingpolyimide for a component, a metal layer like aluminium is preferred inselective etching without etching solder bumps and metalized terminalpads.

Referring to FIG. 5A, Pb-5 wt % Sn solder deposits 11 of 30 μm high wereformed on a solder bump transfer glass plate 1 by using a metal mask 5,wherein in advance to depositing the solder deposits 11, an aluminiumlayer 7 of 0.1 μm thick was deposited on the entire surface of the glassplate 1 except areas for the solder deposits 11 to be deposited, and themetal mask had through-holes of 130 μm in diameter on the first surfaceand 170 μm in diameter on the second surface with an inside wall of atapering angle 100° from the first surface. The second surface of themetal mask was pressed against the surface of the glass plate 1 when thesolder deposits 11 were formed.

Referring to FIG. 5B, after separating the metal mask 5 from the solderbump transfer glass plate 1, a tapering part 32 was unavoidably left onthe aluminium layer 7 around each of the solder deposits 11.

Referring to FIG. 5C, by heating the solder bump transfer glass plate 1at 320° C. in an atmosphere of N₂—H₂ (4:1 in volume), each of the solderdeposits 11 changed into a spherical solder bump on the solder bumptransfer glass plate 1 while the tapering part 32 changed into solderballs 31 on the aluminium layer 7.

Referring to FIG. 5D, by immersing the solder bump transfer glass plate1 into an etchant consisting of 90 ml distiled water, 15 ml HCl, and 10ml HF, the aluminium layer 7 around each of the solder deposits 11 wereremoved together with the solder balls 31 completely. Thus, a solderbump transfer glass plate 1 was obtained which had spherical solderbumps 3 without a tapering part 32 or a solder ball 31 around thespherical solder bumps.

Referring to FIG. 6, a metal mask 5 was a laminated mask consisting ofthe first mask 51 of 50 μm thick 42 -Nickel (Ni) alloy and the secondmask 52 of 50 μm thick 42-Ni alloy. The first and second masks hadconcentric holes of 170 μm and 140 μm in diameters, respectively. Asubstrate 2 for printed circuits had terminal pads 21 of 100 μm indiameters metalized by a triple layer of Au(top)/Ni/Ti(bottom). To forma plurality of solder bumps on the substrate 2, the hole of the mask 5was aligned to the terminal pad pressing the second mask against thesubstrate 2 by a magnetic mask-holder (not shown). Subsequently a solderdeposit of 30 μm in height was deposited on the substrate by vapor phasedeposition of Pb-5 wt % Sn solder through each of the concentric holesover the first mask. Since the solder deposits formed on the substratein concentric holes of the metal mask were not in contact with sidewalls of the concentric holes, none of the solder deposits was found tobe defective after the metal mask 5 was separated from the substrate 2.It made mask-separation without detaching solder deposits possible thata hole of the first metal mask was smaller than that of the second metalmask. For deoxidizing and shaping the solder bumps 3, the substrate 2was coated by solder flux and then heated at a temperature higher than314° C. to melt the solder bumps. After cooling the substrate 2 andwashing the solder flux away, the substrate 2 having a plurality ofspherical solder bumps was completed without a defective bump.

Thus, a flip-chip bonded device will be easily constructed bypositioning the above-completed substrate 2 having a plurality ofspherical solder bumps on a Si chip having Au/Ni/Ti metalized terminalpads and subsequently reflowing the solder bumps in N₂ atmosphericfurnace at a temperature of 350° C.

Referring to FIG. 6, Pb-5 wt % Sn solder in the above example wasreplaced by Indium (In) solder, which changed the bump transferringtemperature from 314° C. to 215° C., and the flip-chip bondingtemperature from 350° C. to 260° C., respectively. Indium solder bumpswas formed on the metalized terminal pads of the Si chip in advance toflip-chip bonding which was actually carried out by bonding the Insolder bumps to each other between the Si chip and the substrate forprinted circuit board, wherein the In solder bumps were bonded to eachother at a temperature of 260° C. in a vapor of fluorocarbon withoutsolder flux.

Referring to FIG. 7A, a metal mask 5 was a laminated 42 Ni-alloy maskconsisting of the first mask 51 having holes of 170 μm in diameter and50 μm in thickness and the second mask 52 having holes of 140 μm indiameter and 50 μm in thickness as referred to FIG. 6. The metal maskwas pressed against a Si substrate 1 for a solder bump transfer platesuch that each of the holes of the first mask was concentrically alignedto the corresponding hole of the second mask by employing a mask holder(not shown) in a solder deposition chamber (also not shown). Pb-63% Snsolder of 30 μm thick was deposited over the metal mask 5 andsubsequently the mask was separated from the Si substrate to form solderdeposits 11 on the Si substrate. Thus, the completed solder bumptransfer plate was obtained without any defective bumps.

Referring to FIG. 7B, another Si substrate 2 was provided to transfersolder bumps from the solder bump transfer plate. The Si substrate 2already had integrated circuits therein and a plurality of Ni/Timetalized terminal pads 21 on a surface of the Si substrate.

Referring to FIG. 7C, the solder bump transfer plate was positioned onthe Si substrate 2 with solder flux 4 such that each of the deposits 11was aligned to the corresponding Ni/Ti metalized terminal pads 21, andthen the whole substrate was heated at a temperature of 250° C. in an N₂atmospheric furnace (also not shown) such that the solder bumps werereflowed to be transferred to the Ni/Ti metalized terminal pads 21.

Referring to FIG. 7D, after the solder bump transfer plate 1 wasseparated from the Si substrate 2, to deoxidize and reshape the solderbumps 3, the substrate 2 was coated by solder flux and again heated at atemperature of 250° C. to melt the solder bumps. After cooling thesubstrate 2 and washing the solder flux away, the substrate 2 having aplurality of spherical solder bumps was completed without a defectivebump.

Referring to FIGS. 7A through 7D, Pb-5 wt % Sn solder in the aboveexample can be replaced by other solders containing In, Bi, Ga, or Sb.The bump transfer plate can be chosen from ceramics and heat resistantpolymer like polyimide instead of Si and glass. The terminal pads can bemetalized by various combination of metal layers such as Au/Ni/Ti orCu/Cr depending upon bump materials.

While the invention has been described having references in particularpreferred embodiments and modifications thereto, various changes in formand detail may be made without departing the spirit and scope of theinvention as claimed.

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 16. A mask having a first surface and asecond surface, opposite to the first surface, for forming solderdeposits onto a surface of a substrate against which the second surfaceof the mask is pressed, comprising: a mask sheet; and a plurality ofthrough holes extending through the mask sheet, a cross-sectional areaof each throughole increasing in steps from the first surface of themask to the second surface while maintaining a similar cross-sectionalshape and centered about a common axis.
 17. The mask according to claim16, wherein the mask sheet comprises: plural laminated layers havingcircular through-holes extending therethrough, each of whichthrough-holes consists of concentric holes having successivelyincreasing diameters, layer by layer, for respective, successive layersfrom the first surface of the mask to the second surface of the mask.18. The mask according to claim 16, wherein the substrate is asemiconductor substrate having a plurality of metalized terminal pads.19. The mask according to claim 16, wherein the substrate is a substratefor a solder bump transfer plate for transferring bumps onto terminalpads on an integrated circuit device.
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 28. The mask according to claim 16, furthercomprising a mask-holder for pressing the second surface of the maskagainst the substrate.
 29. The mask according to claim 16, wherein themask sheet is a laminated mask of nickel alloy sheets.
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